e36 M50(TU) 320 Rough Idle

Ok so I've fitted the standard air box back after aquiring one from a breakers.  It came with a K&N pannel filter which was a bit of a result.  All fitted minus the plastic cowling that fits the front of the box to the scoop in the inner wing/chassis leg (I assume there's some plastic that goes here or the box will still be sucking some hot air?).  I'll try and get the cowling this w/e.  It's definitely made some difference, but the idle is still lumpy.  Looks like I'll have to strip down the vacuum/inlet system to look for leaks/clogged valves.

Anyone got any advise on this process?

Hi Peter,

As far as I am aware all cars pre J plate (and some J plates) did not have to have a CAT.

However in order to pass the emissions test the emissions have to be to the manufacturers specification for the car +/- x% (usually 5%) I believe.  If this is the case then it is possible that a decatted e36 may output higher emissions than a non-CAT e36 as the combined emissions systems are different.

Jayson- unfortunately I live in the countryside and we have way too many "£100 MOT" centres locally.

finite wrote: How does the crank case vent valve cause overfueling?  I haven't checked mine so it's a possibility.

Sorry, no idea. It was just suggested to me as a potential cause of a lumpy idle

finite wrote: My damned brake pad wear light's just come on too (the circle with blocks at the edge) but my pad's look ok!!!

Could be a problem with the sensor.

too true!  or one who will pass a rotten tub bucket for the right money.

shame in many ways.

Have to find a friendly MOT centre I guess.

Any idea how much the emissions go up by?

Ok been doing some research.

Now from what I can make out this is what happens with the CAT/lambda (can anyone confirm if this sequence is correct- ie is the lambda before the CAT?)

air in --> fuel in --> spark --> combustion --> exhaust --> lambda detects level of oxygen and sends voltage reading to ECU --> exhaust gasses go through CAT where some chemical reactions occur to reduce emissions --> exhaust gasses out of tailpipe.

Ok so if the O2 level is too high the ECU ups the injector duty cycle and hence more fuel is used in combustion and vica versa.

From this sequence we can deduce that the lambda/ECU feedback routine would work the same regardless of whethere there is a CAT or not.  Now what it doesn't tell us is how efficient the CAT is and how much it reduced emissions (and hence the smell of fuel out the exhaust).

I've had a look at the Haynes manual which also talks about the crank case emmision control (CC Valve) but goes into no detail about how to replace it/clean it.  It also talks about the charcoal canister and purge valve- both of which could lead to overfueling if they are working incorrectly. Anyone have any experience with these?

Cars! They'd be so simple if it wasn't for government legislation!

Does removal of the CAT cause a lumpy idle?  I can't see how this is possible if the lambda is still in place. 

As an aside, as the car is pre CAT legislation the emmisions test shouldn't be too much of an issue.

Hi Peter,

I didn't have a CAT.  My car was an H reg and had hence pre CAT laws.  Was quite supprised that it didn't being that similar cars of that age usually had CATs.

How does the crank case vent valve cause overfueling?  I haven't checked mine so it's a possibility.

My damned brake pad wear light's just come on too (the circle with blocks at the edge) but my pad's look ok!!!

Anyone have any feedback regarding common faults with the following:

• Lambda sensor
• MAF (hotwire, especially in combination with K&N filter oil)
• Crank Sensor
• Idle Control Valve
• Crankcase Breather Valve
• Throttle Position Sensor
• Coil packs
• BOSCH ECU

Does the M50TU have a air temp sensor?  I find it hard to believe that it doesn't being that it has no Manifold Pressure Sensor!!!

Any other common faults known to lead to lumpy idle?